I don’t know where you live but, where I am, this happens all the time.
Whenever you use Internet (regardless of the speed connection that you’re using) during peak office hours, the Net will tend to slow down. For example, near the end of the month, when most people get paid, the banks are doing all of their monthly balances, transactions, etc., the Net will be a lot slower (for private users and businesses alike).
At night, however, since most businesses are not using Internet, the whole Net will not be as ‘congested’. That’s why most private users will connect at nighttime and on weekends, rather than during the day - because they get better speeds when the companies/businesses are not using Internet.